The present invention relates generally to fuzes for projectiles of various types, and more particularly, to an improved apparatus and method for setting the delay times associated with such fuzes.
In many weapon systems, it is desirable to activate (fire) a group of relatively inexpensive projectiles (rockets or missiles) in rapid sequence. To this end, and as is conventional, each of the rockets or missiles is provided with internal electronic circuitry for establishing a delay following its firing before the rocket or missile is actually armed. Sequencing circuitry, which is also generally known, is then used to separately activate each of the rockets or missiles in desired sequence, whereupon each fired rocket or missile is armed by its respective internal electronic circuitry.
Many of the rockets or missiles which are the subject of this patent application are mass-produced from relatively inexpensive components for purposes of cost-effectiveness. However, the use of inexpensive components leads to the disadvantage that such components will be subject to widely varying values. As a result, it is not uncommon for the actual delay times established by the internal electronic circuitry of such rockets or missiles to vary widely. The inexactitude of fuze delay circuits of this general type has been recognized, and efforts have been made to improve precision.
For example, U.S. Pat. No. 3,986,457 (Mountjoy et al) discloses a fuze setting circuit wherein steps are taken to overcome the deficiencies of imprecise circuit components associated with the projectile fuze (e.g., wide tolerance capacitors) by supplying such circuitry with precision charging voltages (through precision resistors) and by monitoring circuit parameters (i.e., capacitor charging) with comparators for establishing correct levels. However, such circuitry provides no assurances that these correct levels will be established in sufficient time prior to the required detonation of the associated projectile. This is exemplified by alternative means for detonating the projectile in the event that this becomes necessary prior to a complete charging of the fuze setting circuit.
U.S. Pat. No. 3,964,395 (Kaiser et al) discloses a priming circuit for a projectile which can provide extended timing functions making use of capacitors of reduced size. This is accomplished through pulsed operations of the priming circuit, which extends the overall timing function of the circuit to that which would normally be achievable only with unacceptably large components (i.e., capacitors).
However, neither of these measures ensures an accurately set delay time in a conventional fuze delay circuit irrespective of its component values (which can often vary more than .+-.20% from circuit to circuit).
In order to better understand the improvements of the present invention, an explanation of the operation of known projectile fuzes (associated with the rockets or missiles to be fired) is important. For purposes of this discussion, reference will be made to exemplary fuzing for a projectile such as the HYDRA 70, Folding Fin Aerial Rocket (FFAR), which is manufactured by BEI Defense Systems Company, Inc., of Fort Worth, Tex. Such rockets can be equipped with various combinations of motor, warhead and fuze types. Some of the warheads may deploy submunitions or flachettes, and are therefore used with a time delay fuze to allow the warhead to be activated at a precise distance from the launch platform. To this end, fuzes such as the M439 Remote Set Fuze (having a military designation of MIL-F-48877) and the M433 Remote Set Fuze (having a military designation of MIL-F-63281A) are used. The M439 Remote Set Fuze is an analog fuze that provides a time delay function which is proportional to externally applied voltages, and which must be set immediately prior to rocket launch. The M433 Remote Set Fuze is similar to the M439 Remote Set Fuze, except that its components can provide the shorter time delays which are required for warheads that are to be detonated some time following their penetration of a desired target.
Portions of the M439 Remote Set Fuze which are pertinent to these discussions are illustrated in FIG. 1 of the drawings. Operations of this fuze circuit are primarily dependent upon two capacitors. A first capacitor C.sub.t provides the overall time delay function. To this end, the capacitor C.sub.t is positively charged through a diode CR1, to an energy level proportional to the required delay time. A second capacitor C.sub.f provides the energy required to activate the detonator (R.sub.L). To this end, the capacitor C.sub.f is negatively charged through a diode CR2, to an energy level (e.g., of about 1,000 ergs) sufficient to activate the detonator R.sub.L following the set delay time. However, there is a considerable amount of interaction between the capacitors C.sub.t and C.sub.f which must be compensated for in order to achieve a desired timing accuracy. As will be discussed more fully below, a fuze setting circuit is provided to establish the sequence for, and the accuracy of the positive and negative charging of the capacitors C.sub.t and C.sub.f (i.e., the firing sequence).
Activation of the fuze circuit occurs when a Safe/Arm switch SW1 is closed, generally during initial acceleration caused by firing of the motor associated with the rocket. Following this, both of the capacitors C.sub.t and C.sub.f are discharged through the resistors R1 and R2, the transistor Q1, and the (closed) Safe/Arm switch SW1. The characteristics of this discharge are exemplified in FIG. 2. Prior to activation of the fuze circuit (upon closure of the Safe/Arm switch SW1), the voltage across the capacitor C.sub.f exhibits a nominal positive level. Following activation of the fuze circuit, discharge of the capacitor C.sub.f causes a decay in this voltage level. When the voltage across the capacitor C.sub.t falls to zero, the desired delay time has been reached and the transistor Q2 is turned on. This, in turn, toggles and latches a complimentary bi-stable circuit comprised of the transistors Q3 and Q4. This then causes the transistor Q5 to conduct so that remaining charge on the capacitor C.sub.f is applied to the detonator (represented by the load resistance R.sub.L), causing activation of the warhead. The precise delay time established by the fuze circuit of FIG. 1 can be determined by the following equation. EQU T=(C.sub.x *R)*ln(1/(1-((VT-VF)/VT))) (1)
Where:
C.sub.x =1/(1/C.sub.t)+(1/C.sub.f) , PA1 R=R.sub.1 +R.sub.2, PA1 VT=(VC.sub.t +VC.sub.f)-VQ1, PA1 VF=((QC.sub.f -QC.sub.t)/C.sub.f)-VQ1, PA1 QC.sub.t =VC.sub.t *C.sub.t, PA1 QC.sub.f =VC.sub.f *C.sub.f, PA1 VC.sub.t =Voltage across C.sub.t, PA1 VC.sub.f =Voltage across C.sub.f, and PA1 VQ1=Voltage drop across Q1.
Thus, the capacitors C.sub.t and C.sub.f are combined in series to define the capacitance C.sub.x. The delay time is determined by calculating the charge (and thus the voltage) remaining on the capacitor C.sub.f, following discharge of the capacitor C.sub.t. The discharge current of the capacitors C.sub.t and C.sub.f will be identical during the discharge period. As a result, both of the capacitors C.sub.t and C.sub.f will loose an equal amount of charge.
While equation (1) would ordinarily allow for a precise determination of the fuze delay times to be established, the M439 Remote Set Fuze generally employs capacitors having tolerances of .+-.20% (due to cost considerations). As a result of this, the actual delay time established for arming a particular rocket can vary widely. This disadvantage is magnified by the often intended use for such rockets, that being their activation in rapid sequence.